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Simon Peyton Jones authored
There was one place, in type checking parallel list comprehensions where we were unifying types, but had no convenient way to use the resulting coercion; instead we just checked that it was Refl. This was Wrong Wrong; it might fail unpredicably in a GADT-like situation, and it led to extra error-generation code used only in this one place. This patch tidies it all up, by moving the 'return' method from the *comprehension* to the ParStmtBlock. The latter is a new data type, now used for each sub-chunk of a parallel list comprehension. Because of the data type change, quite a few modules are touched, but only in a fairly trivial way. The real changes are in TcMatches (and corresponding desugaring); plus deleting code from TcUnify. This patch also fixes the pretty-printing bug in Trac #6060
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